By Lauren Haworth, LEED AP BD+C
The Dallas-Fort Worth (DFW) Metroplex has experienced rapid growth in recent decades. The population has doubled over the last 30 years, and from July 2014 to July 2015, the area added nearly 150,000 people. While this trend may benefit the Metroplex economically, it can strain existing infrastructure and adversely affect the natural environment. In 2007, the City of Richardson, Texas (an inner suburb of Dallas) was facing both of these issues within a single site on Prairie Creek.
Stream degradation in the DFW area typically occurs as the thalweg, or lowest point, of the stream moves downward until it reaches rock, which is relatively shallow in this area. At this point, the stream spreads outward. Such was the case at this site, and as a result, erosion exposed a sanitary sewer pipe in the streambed, endangering the integrity of the line. The challenge at the Prairie Creek site was to halt or significantly slow the natural outward progression of the stream, as relocating the line was outside the scope of the project.
A preliminary, key component of streambank stabilization projects is determining project extents, both physical and budgetary.
A preliminary, key component of streambank stabilization projects is determining project extents, both physical and budgetary. The appropriate solution often necessitates a delicate balance between cost efficiency and ensuring that the erosion issues are thoroughly addressed. Historically, erosion and flooding issues have been treated using “hard” infrastructure solutions, which typically involved channelizing streams or constructing concrete dams and weirs. These solutions, however, often created other problems, such as reduced water quality, decreased wildlife habitat, and/or higher water velocities.
Traditional, “hard” remedies, or too-short solutions can also merely shift the problem up- or down-stream rather than solving it, causing worse and more costly damage.
In the case of Prairie Creek, the stabilization efforts needed to encompass 580 linear feet of the stream in order to protect the exposed pipe and prevent further exposure that could result from future erosion.
Streambank restoration seeks to undo prior damage by returning waterways to their more natural state, which can be as simple as removing a concrete weir or as complex as designing a new wetland. The goals are often to protect property and infrastructure by slowing stream velocities, improving water quality, providing habitat for fish and other wildlife, and reducing erosion and downstream sedimentation.
To protect the exposed pipe at Prairie Creek, provide a cost-effective solution, and respect the natural environment of the stream, a “soft” solution was implemented. The bank of the stream closest to the sanitary line was regraded to follow a gentler slope, making it more stable. Where possible, the grading was designed to protect the many existing trees. The exposed sanitary line was re-encased in concrete, and the slopes were then armored with erosion control mats which were anchored into the concrete encasement to ensure their longevity. The erosion control mats, along with the gently banked slope, made it easier for vegetation to take root and flourish. Vegetation is crucial for maintaining the stability of a slope, as root systems help hold erodible soil in place.
Construction at Prairie Creek was completed in 2011. In the course of a later project — a 2014 erosion study conducted on several of Richardson’s streams — the Prairie Creek site had integrated so well within the surrounding landscape that the engineers conducting the inspection (who were not part of the original project) were unable to immediately identify where the improvements had been made. The anchoring system for the erosion mat successfully held it in place, allowing vegetation to take hold and return a more natural appearance to the stabilized stream.